There remains controversy over the effects of dietary fat content on energy intake and body fat mass in adult humans. In addition, little is known about the extent to which genetic inheritance influences the effect of dietary fat content on energy intake and energy balance. A pilot study in monozygotic twins will be conducted to address these important tissues. The specific hypotheses to be tested are: 1) that the proportion of dietary energy provided by fat exerts a significant influence on energy intake and energy balance even when other factors that influence food intake are standardized; 2) that there is a significant familial influence on the ability to maintain energy balance during consumption of diets with different fat contents; and 3) that the effects of dietary fat on energy intake are mediated in part by factors influencing the extent to which substrate oxidation can adjust to maintain carbohydrate balance. The subjects will be 8 pairs of monozygotic twins aged 20-30 years. They will be studied during two 10-day residency periods at the Human Nutrition Research Center on Aging at Tufts University and also over 7 days while leading normal lives at home. During the two residency periods, subjects will be provided with diets containing 20% and 40% of energy as fat in a cross-over design. These diets will be standardized for palatability, energy density, and fiber and protein contents, and subjects will be blinded to variations in dietary fat content. Voluntary nutrient intakes on these two dietary regimens will be measured exactly. In addition, during each residency period, total energy expenditure will be determined using the doubly labeled water method, substrate oxidation will be determined using indirect calorimetry, and body fat mass will be determined with the total body carbon technique. While the subjects live at home, dietary intake will also be studied, using 7-day food intake diaries. Using repeated measures analysis of variance the effect of dietary fat content on energy intake and energy balance (calculated as the difference between metabolize energy intake and total energy expenditure) will be determined and intraclass correlation coefficients will computed to assess the contribution of within-twin-pair similarities to the overall variance in changes in energy intake and energy balance with the dietary regimen. These studies will provide initial estimates of the effect of dietary fat intake on energy intake in humans and will also indicate the susceptibility to overeating on high fat diets is a familial trait. They will also provide a basis for developing further detailed investigations to quantify the influence of dietary composition-genotype interactions on energy balance.